1. Field of the Invention
This invention is related to a tetrafluoroethylene/hexafluoropropylene copolymer and the production method thereof, and an electrical wire.
2. Background Information
A fluororesin has excellent characteristics such as thermal resistance, chemical resistance, solvent resistance, insulation properties and the like. For this reason, the fluororesin is molded into various products such as tubes, pipes, filaments, and the like by melt extrusion molding and the like, and the products are commercially available. In particular, tetrafluoroethylene (abbreviated to “TFE” in the following)/hexafluoropropylene (abbreviated to “HFP” in the following) copolymer (abbreviated to “FEP” in the following) has a lower dielectric constant, a lower dielectric loss tangent and excellent insulation properties than other fluororesins. Therefore, FEP is preferably used for coating of an electrical wire such as a cable, wire and the like.
By the way, improvement of productivity and reduction of cost and the like are currently required in the manufacturing setting for coating the electrical wire and the like. Consequently, various considerations to improve molding speed and to reduce defects are proposed in the manufacturing setting. For instance, polytetrafluoroethylene (abbreviated to “PTFE” in the following) is added to FEP so that from 0.01 to 5 wt % of PTFE is contained in the whole polymer to improve melt fracture phenomena and critical extrusion speed (for example, refer to Japanese Published Unexamined Patent Application No. S52-98761 (1977)), from 0.03 to 2 parts by weight of PTFE is added to 100 parts by weight of FEP or the terpolymer consisting of TFE, HFP and perfluoroalkylvinylether (abbreviated to “PAVE” in the following) to improve coating broken during wire coating process (for example, refer to International Patent Publication Nos. WO 03/22922 and WO 03/22923 pamphlets). These techniques can reduce the size of a lump which is produced between a coating resin and a conductive wire in some degree but they can't reduce the occurrence frequency of the lump substantially.
Moreover, a melt-processable fluororesin composition containing from 0.01 to 3 parts by weight of PTFE with a standard specific gravity of from 2.15 to 2.30 to 100 parts by weight of FEP is prepared to improve moldability in melt extrusion molding, especially to reduce defects significantly in high-speed extrusion coating of an electrical wire (for example, refer to International Patent Publication No. WO 123694 pamphlet). Furthermore, for this melt-processable fluororesin composition, it is necessary to mix PTFE in dispersion liquid with FEP in dispersion liquid.
Additionally, a melt-processable fluororesin composition containing from 0.01 to 5 wt % of a perfluoropolymer to FEP with high molecular weight and with a melting point of over 20 degree C. higher than that of FEP is prepared (for example, refer to Published Japanese translation of a PCT Application No. 2004-502853). Note that it is necessary to mix the perfluoropolymer in dispersion liquid with FEP in dispersion liquid for the melt-processable fluororesin composition. Furthermore, an example of the perfluoropolymer includes FEP with from 2 to 20 wt % of a repeating unit derived from HFP. It is reported that this melt-processable fluororesin composition can remove a lump and can depress piling the lump up in extrusion coating of an electrical wire. However, this melt-processable fluororesin composition can't reduce the occurrence frequency of the lump substantially as well as the melt-processable fluororesin in which PTFE powder is mixed with FEP powder. Moreover, fluctuation of capacitance is larger and the electrical properties of the final products made from the melt-processable fluororesin composition are worse when a smaller lump is produced.
Furthermore, FEP with a relatively high die swell of from 5 to 20% and with a melt flow rate within a specific range is prepared to reduce nonuniformity of a diameter of electrical wire in extrusion coating of the electrical wire (for example, refer to International Patent Publication WO 01/36504 pamphlet). However, a perfluoropolymer with high molecular weight, such as PTFE and the like is not added to this FEP. Additionally, the technique for reducing the occurrence frequency of the lump is not mentioned.
TFE/fluoroalkoxytrifluoroethylene copolymer (abbreviated to “PFA” in the following) composition containing from 0.01 to 30 wt % of PTFE with a crystallization temperature of more than or equal to 305 degree C. is prepared as a fluororesin that can be a raw material for a molded body with an excellent surface flatness (for example, refer to Japanese Published Unexamined Patent Application No. H7-70397 (1995)). However, it is not clear that this PFA composition can give an electrical wire without any defects in high-speed extrusion coating of an electrical wire.